With the upcoming release of the Windows 10 Anniversary Update, the Release Preview ring is moving to Windows 10 Mobile Insider Preview build 14393. This change will have some impact on unsupported devices:

As stated before, unsupported devices will not receive any builds or updates newer than 10586.x

Unsupported devices will not be able to select any Windows Insider Preview rings in the Windows Insider app

Updates to apps and services may not be available for unsupported devices and in some cases may limit certain experiences